The present invention relates to a process and an apparatus for continuous determination of the states of anisotropy of optically active materials by means of their double refractions in one or more principal directions, wherein the path difference of interfering wave trains is generated with the aid of an optical compensator, with the thickness of the optically active material being measured simultaneously and an interference pattern generated by the compensator being evaluated photoelectrically as a function of position.
German Offenlegungsschrift No. 24 49 475 teaches such a process for determining the state of anisotropy of transparent or translucent polymers by measuring the orientation double refraction employing a compensator, which provides the path difference required for determining the double refraction. Photelectronic analysis of the interference color pattern is employed to measure the path difference, said pattern being obtained behind an analyzer connected to the compensator, which is a wedge compensator. The path difference, which appears as an optically visible signal, is converted into an electrical signal with the aid of photosensitive components, photocells for example. At least two beam paths are evaluated to determine the degree of anisotropy, one of said paths preferably penetrating the sample under test vertically and the other, diagonally. The electrical signal corresponding to the optical path difference can be used to control the manufacturing process or for semi-continuous quality control with computer control. The device for measuring the orientation double refraction comprises a position-sensitive stripe detector which measures the total light intensity striking it with simultaneous indication of the location of maximum brightness. The stripe detector records the interference pattern obtained by the compensator. Instead of a stripe detector, a multiple detector can also be used in which the individual detectors are interrogated by means of a control logic individually as a function of the light intensities received from the interference color pattern and the detector, which receives the maximum intensity, shows the path difference or the phase shift through the optically active material through which the light passes. Similarly, one or more photosensitive elements can be provided to measure the optical path difference, said elements receiving the interference color pattern and being moved electromechanically by means of the compensator to determine the location of maximum brightness with parallel polarizers or maximum darkness with crossed polarizers.
The compensating wedge used in this process as a compensator is known. The compensating wedge covers a certain range of the path difference and generates a black stripe when traversed by light with crossed polarizers and a white stripe when the polarizers are parallel, said stripe being referred to hereinbelow as the interference stripe of zero-th order. This stripe is flanked to left and right by interference color stripes of the zero-th to nth order. The position of the individual interference stripes can be read off, for example using a scale division in nanometers, mounted on the commercially available compensating wedge. When the compensating wedge and the optically active material, which simultaneously exhibits double refraction properties, are fitted together, the path differences of the compensating wedge and the material are added together so that the interference stripes of zero-th order is displaced on the compensating wedge. This local displacement is a direct measure of the path difference in the material. As far as continuous measurement of anisotropic properties is concerned, this poses the difficulty that inhomogeneities generally appear in optically active material such as biaxially stretched films, which can also indicate the start of tearing of the film material. These inhomogeneities then frequently produce a path difference which is so great that, as a result of the addition of this path difference to the path difference of the compensating wedge, the interference stripe of zero-th order migrates out of the measurement range of the compensating wedge. In practice the interference pattern on the compensating wedge may not contain any interference stripes of zero-th order. In such a situation the detector means--the stripe dector, the multiple detector, or a photosensitive element moved electromechanically over the compensating wedge--merely detect an interference color stripe (if there is in fact an interference color stripe) which is significantly less bright than the interference stripe zero-th order. Such an interference color stripe provides no information about the double refraction of the material. It is therefore obvious that, with the known method, the danger of tearing of a film web which is indicated by pronounced changes in the double refraction or the path difference, cannot be measured continuously and can only be detected within very narrow limits in order to counteract it in proper time. If the range of the compensating wedge is increased to measure the path difference in order to overcome this disadvantage, there will be considerable deterioration of the resolution of the individual interference stripes so that the photosensitive detectors are frequently incapable of pinpointing the location of maximum brightness or maximum darkness within the interference color pattern on the compensating wedge and therefore the path difference or double refraction cannot be calculated.
German Patent No. 23 38 305 teaches a method for determining linear double refraction in an optically active material, wherein the material is irradiated by linearly polarized light and the emergent light is detected in a polarization plane perpendicular to the polarization plane of the incident light, whereby at least one wavelength is measured in which the detected light is extinguished. The measuring device used for this purpose comprises a light source whose beam passes through a polarizer in which the required linearly polarized wave is generated, which then passes through the film to be measured and enters an analyzer, thence emerging into a detector system. The detector system can be designed as a prism or grating or an optical multichannel analyzer with a plurality of detectors. The light source emits monochromatic or white light. A compensating wedge for measuring the path difference created by the film is not provided. Finally, German Offenlegungsschrift 31 06 818 teaches a method for continuous determination of multiaxial orientation states of stretched films or plates by means of their principal double refraction values, wherein three laser beam paths are employed, generated by three lasers or by splitting one laser beam, one of which passes perpendicularly through the film and the others pass through it at inclinations such that the inclination planes are perpendicular to the film surface and contain the two main orientation directions. The phase differences of the laser beam intensities are continuously measured after passing through the film, a quarter-wave plate, and a rotating analyzer. The three main double refraction values of the film are measured continuously from the three phase differences, considering the two slope angles of the sloping laser beams and the film thickness, measured in another fashion. The measuring device comprises three lasers directed parallel through a suitable system of lenses or mirrors after passing through the film. The light beam from one laser strikes the film plane perpendicularly, while the beams from the other two lasers strike the film at an angle .phi. to the normal of the film. The light beam of one laser runs in a plane which contains the film normal and the principal stretching of the film, while the light beam from the other laser lies in a plane which is determined by the film normal and the transverse stretching direction of the film. The optical anisotropy of the stretched film elliptically polarizes the light beams emerging from the lasers which are initially linearly polarized. A quarter-wave plate beneath the film converts the elliptical polarizing of the three laser beams into a linear polarization. A rotating polarizing filter beneath the quarter-wave plate cancels out the beams when their polarization directions are perpendicular to the polarization direction of the polarization filter. The intensities of the laser beams are converted into periodic electrical signals by photosensitive detectors, said signals being phase-shifted with respect to one another. These phase shifts can be determined with the aid of two phase meters, with the third phase shift supplementing the other two to 0.degree.. The double refraction values can be determined from the measured phase shifts in a computer and used directly as parameters for biaxial film orientation for controlling a film stretcher.
This known measuring device is costly to build because it uses three lasers or an optical system to split a single laser beam into three beams.
An object of the invention is to provide a measuring method and a measuring apparatus with which it is possible continuously to determine the double refraction values of an optically active material such as a mono- or biaxially stretched film and to detect the danger of imminent tearing of the film in proper time.